(a) Field of the Invention
This invention relates to an improved polyisocyanate composition and a process for the preparation thereof. More particularly it relates to a polymethylene polyphenyl polyisocyanate composition containing up to 10 percent by mole of an alkoxycarbonylamino group (hereinafter referred to as a carbamate group) based on an isocyanate group in the composition, and to a process for the preparation thereof.
(b) Description of the Prior Art
A methylene crosslinked polyphenyl isocyanate reacts with polyfunctional hydrogen compounds, such as polyesters, polyester amides and polyethers, to form rigid polyurethane substances. Such reaction can be conducted in the presence of a blowing agent to obtain a foam product. The polyurethane foam thus obtained is generally called a rigid foam and has a closed-cell structure with excellent heat insulating properties, heat resistance, weatherability and chemical resistance. The foam has various uses, for example, heat insulating materials for use in a refrigerator, a house and the like, lagging materials for use in oil tanks, chemical plants and etc., and the like. Recently, however, there has been an increased demand for the weight saving of the foam for the saving of energy and resources it provides.
However, a reduction of the foam density to obtain the weight saving of the rigid polyurethane foam causes a lowering of the compressive strength, degree of closed-cell and dimensional stability. The reduction of the density can readily be obtained by increasing the amount of the blowing agent, but it causes a lowering of the compressive strength. Accordingly attempts have been made to increase the compressive strength by use of sugar base polyethers or sugar amine base polyethers as the polyol; but the dimensional stability at lower temperatures is decreased to a level which is unsatisfactory for use in refrigerators and the like due to the above-stated drawback. Further, an improvement has been proposed whereby a catalyst is selected to regulate the polymerization rate and expansion rate, but unsatisfactory results are obtained.